The present invention relates to a bandpass filter for a radio-frequency signal, in particular in microstrip technique, and a method for tuning the transmission band of such a filter. Filters of this type are known from a number of documents, among which U.S. Pat. No. 5,825,263 and U.S. Pat. No. 5,786,303 are cited here as examples.
Conventional filters of this type, described as prior art in U.S. Pat. No. 5,825,263, are formed of a plurality of conductor segments having a length of xcex/4 or xcex/2 structured on a substrate in a staggered configuration, wherein adjacent segments overlap with each other over a length of xcex/4, xcex being the wavelength corresponding to a center frequency of the passband of the filter.
FIG. 1 shows such a filter having an input segment 1 and an output segment 3 of length xcex/4 connected to signal lines, and resonator segments 2 of length xcex/2 in between.
The operation principle of this filter relies upon the fact that the resonator segments 2 are consecutively excited to oscillate in their fundamental mode by a radio-frequency signal applied to input segment 1 and having a wavelength xcex which is two times the length of resonator segments 2. These oscillations, in turn, induce the filtered signal in output segment 3.
The electric currents flowing in the segments induce electromagnetic fields around the segments. While the fields in the substrate plane are necessary in order to excite adjacent segments, the energy contained in fields outside the substrate plane is lost. This causes strong losses of the filter, unless a screening is provided which reflects fields radiated off the substrate plane back to the segments.
However, this screening is no completely satisfying solution to the loss problem. The distance between segments and screening cannot be made zero. In consequence, a phase shift between the currents flowing in the segments and the fields reflected back to the segments by the screening is unavoidable. This leads to a displacement of the transmission frequency of the filter that depends on the distance between the segments and the screening and on the dielectric constant of the material in between. Therefore, it is difficult to produce radio-frequency filters of the type shown in FIG. 1 having a precisely predefined, desired transmission frequency. If a filter having a precisely specified transmission frequency is needed, the only practical possibility is to select, from a large number of finished filters having different center frequencies due to manufacturing scatter, those that have exactly the desired value.
In order to solve the problem of excessive radiation off the substrate plane, U.S. Pat. No. 5,825,263 suggests a filter arrangement which is essentially formed of two pairs of filters, each of which is formed of staggered resonator segments similar to those of FIG. 1, wherein one filter is a mirror-image image of the other. The two inputs of this filter pair are supplied with a balanced input signal, so that in corresponding segments of the filters, currents are flowing in opposite directions at all times. The fields radiated by these currents cancel out on the plane of symmetry between two filters and thus reduce the radiation perpendicular to the substrate plane.
In order to operate such a filter arrangement with an unbalanced signal, it is necessary to provide a balun upstream and downstream of the individual filters for transforming the asymmetric signal into a symmetric, balanced signal and the filtered, balanced signal back into an asymmetric signal, respectively. This prior art filter arrangement is therefore expensive in manufacturing and requires a large substrate surface.
The object of the present invention is to provide a filter for radio-frequency signals having small radiation loss and, at the same time, a simple, space-saving structure.
The object is solved by a bandpass filter having the features described herein.
By the resonator having a binary (rotation or mirror-image) symmetry and being excitable by the signal to be filtered into a resonance having the same symmetry, it is achieved that a current excited in the resonator spreads out symmetrically in the resonator from a fixed point of the symmetry operation. Accordingly, at all times currents having the same amplitude and opposite polarities exist within the resonator at opposite sides and in equal distances from the center of symmetryxe2x80x94the plane of symmetry, if the binary symmetry operation is a mirror-image reflection, or the axis of symmetry, if the symmetry operation is a 180xc2x0-rotation-, the radiation fields of which cancel out on the plane of symmetry or axis. This effect is achieved without before having to convert an asymmetric signal into a balanced signal using a balun.
According to a first preferred embodiment, the bandpass filter comprises at least two resonators, of which one is directly coupled to the input section and the other is directly coupled to the output section. Between said two resonators, further resonators can be provided. Preferably, all these resonators are symmetric with respect to the same mirror plane.
According to a second preferred embodiment, the input section and the output section each comprise a sending electrode for exciting a resonator and an input conductor connected with the sending electrode and/or a receiving electrode to be excited by the resonance of the resonator and an output conductor connected to the receiving electrode, respectively.
Electrodes and resonators are preferably not directly coupled, so that only a capacitive or magnetic coupling is possible between the two.
Said two embodiments can be combined by the electrodes simultaneously being resonators.
Input and/or output conductors preferably extend at right angles with respect to the input and output electrode, respectively. By this arrangement of the input and output conductors, an influence of the fields generated by the conductor on the current distribution in the corresponding electrode is prevented.
Preferably, all resonators have the same extension transversally to the plane of symmetry. This extension corresponds to the entire wavelength xcex of the resonance frequency of the resonators.
This feature, and more specifically a perfect congruence of the resonators facilitates tuning the bandpass filter of the present invention to a desired resonance frequency, as will become more evident later on.
Preferably, the resonators are elongated transversally with respect to the plane of symmetry. Such a shape enables a very low loss coupling. Considering available space, one might also consider an angled or curved form of the electrodes and the resonators, however, in case of mirror-image symmetry it must then be accepted that the radiated fields no longer cancel out completely in the plane of symmetry.
In an embodiment of the bandpass filter which is particularly simple to manufacture and has low loss, each resonator has a constant cross section area perpendicular to the plane of symmetry.
Alternatively, each resonator may have a constriction in a section between the plane of symmetry and each of its longitudinal ends. This feature has the advantage that the extension of the resonator perpendicular to the plane of symmetry at constant resonance frequency is shortened with respect to the alternative considered above, so that the required area for the bandpass filter can be reduced.
Due to the low radiation, the bandpass filter of the present invention is also operable without a screening enclosing the resonator, and/or its transmission behaviour depends little on such a screening and on the dielectric constant of a material provided between the filter and the screening. Thus it becomes possible to tune such a filter after structuring to a predefined, desired transmission frequency band by removing material from the resonator while maintaining its symmetry. A practical way of carrying out such a removal is laser ablation.